The present invention concerns digital transmission of information. Higher-order coding is provided for better utilization of transmission paths. Such coding has been done by line conduction, by wireless routes and over optical fibers.
Known methods in this regard include ASK (amplitude shift keying), FSK (frequency shift keying) and PSK (phase shift keying). Recently methods have also been disclosed where the phase relation of pulses (German Patent No. 4,345,253, U.S. Pat. No. 5,587,797) and the pulse duration (PCT/EP97/07196-WO98/14042) have been used as coding levels in that an alternating current of a frequency and a phase relation with a predetermined number of periods has been used for this purpose, and the levels were changed by increasing or decreasing the number of periods.
The object of this invention is to provide a method whereby it is possible to increase the coding levels with simple means, namely for codes where the code elements consist of an uninterrupted periodic sequence, e.g., with an alternating current of the same frequency, and where the following code element is differentiated by a change in amplitude. An increase in the number of levels is achieved by flagging the beginning or the end of the amplitude code element marking with positive or negative elements, e.g., with a positive or negative half-period. If the periodic sequence is an alternating current, it can also be provided directly as a transmission alternating current, i.e., no separate carrier is then necessary. The method according to this invention is apparent from Patent claim 1.